The Realm of Atlos
The Underdark is the name universally given to a vast subterranean network of interconnected caverns and tunnels, stretching beneath all of Atlos and forming an underworld of sorts.
The vast subterranean realm is inhabited by drow, mindflayers, aboleth, and other strange, sinister creatures. It is a place where few humans go and from where even fewer return. It extends well past the dungeons made by surface dwellers, encompassing myriad caverns, tunnels and complexes.
There is no light except for occasional patches of phosphorescent fungus; most Underdark inhabitants either have highly developed senses other than sight or have developed darkvision. Food can be extremely difficult to find, and much of the natural vegetation is poisonous. In addition, potable water is hard to locate.
There is no unified underground government since each individual city-state has a different form of rule. The Underdark economy deals primarily in armor, exotic goods, magic, slaves, timber and weapons. The ethical code of many indigenous races tends toward evil or neutral.